Circuits for protection against overvoltages are typically used in electronic devices that are supplied with electrical energy via supply terminals. Electronic automation devices in particular are, for example, supplied with DC voltage of +24 V. Various events, such as switching from inductive loads, lightning strikes or interference voltages being coupled in through electric lines, can lead to high-energy interference pulses (surges) being coupled into the supply voltage. The pulse voltage of such interference pulses or overvoltage pulses is very high by comparison, and can lead to electronic components in electronic devices being destroyed if suitable protective measures are not taken.
Electronic circuits therefore have to be protected against this type of fault to prevent components being destroyed. In industrial settings, overvoltage pulses of e.g. 500 V for a DC supply are specified by relevant standards (for example, the standards IEC/EN 61131-2 or IEC/EN 61000-6-2). When coupled in via an intermediate resistance of 2 Ohm, in theory this can cause current pulses of up to 250 A.
To protect electronic components from overvoltages of this type, circuits of the above type are known. Varistors, thyristors or suppressor diodes are used as default overvoltage protection elements. These components conduct at least part of a current pulse away towards a reference potential and restrict an overvoltage pulse to a considerably lower level. Depending on the type of component, overvoltage pulses of for example up to approximately 50 to 60 V (in suppressor diodes) or approximately 100 V (in varistors) can be suppressed.
In protective circuits according to the prior art, an embodiment of which is shown in FIG. 1 by way of example, additional filter elements, such as a series filter coil L1 and an electrolytic buffer capacitor C1, are used in addition to suppressor diodes (cf. V2 in the protective circuit S in FIG. 1). The buffer capacitor C1 also filters an overvoltage pulse u1 at an input 1 of the circuit so that the actual level of an interference pulse is reduced further. Downstream electronic components (see voltage transformer NT in FIG. 1 or other components that can be powered at an operation side 3 by means of a VCC supply voltage u3) are protected as a result.
In the case of very small or compact devices having a very limited amount of installation space, or when an electrolytic capacitor with sufficient capacity for filtering an overvoltage pulse is either not present or unable to be used for reasons of space, the use of a suppressor diode may not be sufficient in some cases for completely suppressing an overvoltage pulse or for preventing destruction of sensitive components. In such cases, an additional protective measure is required.
It is known from the prior art to use a field-effect transistor in a series path of the protective circuit, which transistor switches on when a particular input voltage level is reached and thus protects the electrically downstream parts of the circuit. Conventional solutions are disadvantageous in that an input voltage of the protective circuit has to be significantly higher than the output voltage for the field-effect transistor to be open for normal functioning. Specifically, this may, for example, imply a voltage loss of approximately 3 V. In addition, the voltage loss of the field-effect transistor becomes very high, even with relatively high rated currents (for example from approximately 3 to 4 A). Other solutions from U.S. Pat. No. 8,068,321 B2 and CN 203415972 propose very specific transistor designs and complex controls for the protective function, respectively, which however can be complex and unfavourable in practice.